1. Field of the Invention
This invention relates to a magnetic trip device for circuit breakers and, more specifically, to a support structure which maintains the alignment of a plunger in the magnetic trip device.
2. Description of the Prior Art
Molded case circuit breakers are well known in the art as exemplified by U.S. Pat. No. 5,927,484 to Malingowski issued Jul. 27, 1999 and by U.S. Pat. No. 4,503,408 issued Mar. 5, 1985 to Mrenna et.al., entitled xe2x80x9cMolded Case of Circuit Apparatus Having Trip Bar With Flexible Armature Interconnectionxe2x80x9d assigned to the assignee of the present application. The foregoing are incorporated herein by reference.
In molded case circuit breakers in which the power contacts, operating mechanism, and trip unit are mounted inside of a molded plastic insulative housing, a common type of magnetic trip device is a solenoid which includes a stationary core through which the current in the protected circuit is passed. The current passing through the stationary core creates a magnetic field. When there a very high instantaneous currents, such as those associated with a short circuit, the magnetic field intensifies. A plunger assembly, having a movable core and a plunger tab which engages the trip latch on the operating mechanism, is partially disposed within the stationary core. Typically, a spring provides a limited force biasing the movable core away from the stationary core and preventing the plunger from engaging the trip latch. The force of the spring is overcome by the magnetic field generated by the stationary core during a short circuit. That is, when a short circuit occurs, the current in the stationary core creates a magnetic field strong enough to overcome the moveable core spring thereby allowing the moveable core to move toward the stationary core and causing the plunger to engage the trip latch.
The amount of current required to trip the device can be controlled by adjusting the amount of separation between the plunger assembly and stationary core. When the plunger assembly is located closer to the stationary core, a weaker magnetic field, and therefore a lower current, is required to draw the plunger assembly toward the stationary core to trip the device. In order to adjust the trip condition, the plunger assembly is mounted in a plunger assembly support structure having a base and a moveable plunger carriage. The carriage allows the plunger assembly, including the moveable core, to be moved relative to the stationary core. A carriage is used so that adjusting the gap between the moveable core and the stationary core does not impact on the compression of the moveable core biasing spring. The moveable plunger carriage is coupled to an adjustment mechanism to address the initial gap between the plunger assembly and the stationary core.
Plunger assembly support structures of the prior art were loosely disposed within plunger carriage cavities in the circuit breaker housing. Such support structures did not include a means to maintain the alignment of the plunger carriage relative to the base or the stationary core. As such, the plunger carriage could wobble in the plunger carrier cavity, resulting in a mis-alignment of the plunger assembly. Additionally, through repeated use, the moveable core on prior art magnetic trip devices can rotate allowing the plunger tab to move away from the trip bar actuator arm.
There is a need, therefore, for a plunger assembly support structure for a molded case circuit breaker magnetic trip mechanism which maintains the orientation of the plunger carriage in the circuit breaker housing.
There is a further need for a plunger assembly support structure for a molded case circuit breaker magnetic trip mechanism which provides a means for maintaining the moveable core alignment with the stationery core.
There is a further need for a plunger assembly support structure for a molded case circuit breaker magnetic trip mechanism which corrects rotation of the plunger tab on the moveable core.
These needs and others are satisfied by the invention which provides a plunger assembly support structure for a magnetic trip unit which includes a plurality of guides which maintain the orientation of the plunger carriage. This invention further provides a plunger guide which corrects rotation of the moveable core of the solenoid.
A molded case circuit breaker includes at least one pair of separable main contacts. The main contacts are disposed in the circuit breaker housing. The circuit breaker may be tripped manually by a handle or by a magnetic trip device. The magnetic trip device includes a rotating trip bar, which is actuated by a plunger assembly, a moveable plunger assembly and a stationary core. The stationary core is in disposed between, and in electrical communication with, the main contact and the load side of the circuit breaker. The plunger assembly includes a moveable core, partially disposed within the stationary core. The plunger assembly is disposed within a plunger assembly support structure having a base assembly and a movable plunger carriage. The base assembly is coupled to the circuit breaker housing. The plunger carriage is coupled to the base. The base includes a plurality of guides which align the plunger carriage and plunger with the trip bar. The base assembly further includes a conical indentation which reorients the plunger if it rotates in the plunger carriage.